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| Albedo
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| Definition |
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Albedo is the fraction of radiation that is reflected by the earth's surface.
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| Rationale |
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Surface albedo is fundamental to surface energy balance, as it is the ratio of the downwelling to upwelling short-wave radiative fluxes at the earth's surface. Surface albedo and clouds are the two most important parameters modulating the earth's climate, especially over land. Although it does not change temporally as much as clouds do, surface albedo undergoes considerable variation over space, and it shows strong seasonal cycles and inter-annual variability. Surface albedo data have been obtained from in situ observations (Ohmura and Gilgen, 1991) and from space-borne measurements by means of remote-sensing techniques (Li and Garand, 1994). The former are site-specific as they usually represent very small areas but have high temporal resolution, whereas the opposite is true for the latter. Therefore, the two types of measurements are complementary.
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| Users |
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Modellers and analysts concerned with climate, weather, ecosystem, and hydrological issues.
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| Assessment method |
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Tiers 2-3: tower-mounted hemispheric sensors;
Tier 5: satellite measurements.
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| Units of Measure |
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No unit
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| Frequency of measurement |
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10 min means at centres or experimental sites, hourly means at stations, monthly means from space.
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| Spatial resolution |
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The measurements of albedo should be taken by hemispheric sensors mounted, as high as possible, on towers. The measurement locations should be chosen within relatively homogeneous regions. As a result, the spatial resolution of the surface observations can vary. The spatial resolution of satellite-based albedo measurements depends on the field of view of the satellite sensors, ranging from 1 km for operational meteorological satellite to tens of kilometres for meteorological research satellites. The data can be degraded to the larger grids required for models (typically 250 km for GCMs at the present).
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| Accuracy/precision required |
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Relative accuracy ± 10%;
Absolute accuracy better than ± 5%.
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| Associated measurements |
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Upwelling and downwelling short-wave radiative fluxes at the surface, short-wave radiative flux at the top of the atmosphere, sky condition, and atmospheric parameters (water vapour, aerosol).
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| Present status |
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Current global data sets on surface albedo have a relative discrepancy of 20-30% in low- and mid-latitudes and over 50% in polar regions (Li et al., 1997). There exist essentially two types of global surface albedo data sets. One is obtained by combining global cover type information with the typical albedo values obtained for each cover type (Wilson and Henderson-Sellers, 1985). Such data sets generally do not contain sufficient temporal resolution and lack information on the interannual variability. The other is derived from clear-sky satellite measurements (Staylor et al., 1990; Li and Garand, 1994), which are hindered by the presence of clouds and insufficient knowledge of the atmospheric state. The major limitation for both types of data is the lack of spectral variation that is required by most models.
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| R and D needed |
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- Continue the development of satellite-based inversion algorithms with the focus of retrieving spectral surface albedo; improve the algorithms for cloud detection, angular, spectral and diurnal corrections; synergistic use of ground and spaced-based data to provide a global albedo data set of high spatial and temporal resolution.
- Promote the installation of towers mounted with downward- and upward-looking, hemispheric short-wave radiometers;
- Promote the acquisition of global aerosol data sets.
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References |
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Li, Z. and
Garand, L. 1994. Estimation of surface albedo from space: A
parameterization for global application. Journal of Geophysical Research,
99, 8335-8350.
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Li, Z.;
Moreau, L.; Arking, A. 1997. On solar energy disposition. A perspective
from observation and modelling. Bull. Amer. Meteor. Soc., 78, 53-70.
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Ohmura, A.
and Gilgen, H. 1991. Global Energy Balance Archive (GEBA). World Climate
Program - Water Project A7, Rep. 2: The Geba Database Interactive Application,
Retrieving Data. Zurich, Verlag der Fachvereine, 60 pp.
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Staylor,
W.F. and Wilber, A.C. 1990. Global surface albedo estimated from ERBE
data. Proc. 7th AMS Conf. Atmos. Rad., July 23-27, 1990, San
Francisco, CA, 231-236.
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Wilson,
M.F.; Henderson-Sellers, A. 1985. A global archive of land cover and soil
data for use in general circulation climate models. Journal of Climatology,
5, 2, 119-143.
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